On the magnitude of the stratospheric radiative feedback in global warming

Yi Huang, Yuwei WangDepartment of Atmospheric & Oceanic Sciences

McGill University

EGU 2020AS3.3

2020.05.07

GCM-projected moistening in global warming [Huang et al. 2016]

Dlog2(q)2

-2

Stratospheric radiative feedback

• Feedback response: those driven by surface warming, e.g., abrupt4xCO2-sst4xCO2

• Significant stratospheric radiative feedback?– Significant moistening -> SWV feedback

[Forster & Shine 2002, Solomon et al. 2010, Dessler et al. 2013, Banerjee et al. 2019…]: FDH method estimates 0.2-0.3 W m-2 K-1

– Small overall stratospheric radiative feedback: SWV radiative forcing + Strato. Temperature change induced radiative flux change ~ O(0.01 W m-2 K-1) [Huang et al. 2016]

SWV: Stratospheric Water VaporFDH: Stratospheric radiative equilibrium and TOA flux change simulated under Fixed Dynamical Heating.

Agreements and disagreements

• TOA radiation budget perspective• SWV radiative effect = Fdirect + Findirect

Fdirect : SWV direct forcing (greenhouse effect: trapping of OLR)Findirect : SWV indirect forcing via cooling strat. temperature and reducing OLR

• Agreed– Fdirect : small, O(0.1 W m-2) [Huang et al. 2016; Banerjee et al. 2019]– Findirect(Strat. Temp.) via FDH: large, O(1.0 W m-2) [Banerjee et al. 2019, …]

• Disagreed – Is feedback temperature response in stratosphere dominated by SWV?

Overall DTstratos

Hypothetical (FDH-based) SWV-caused DTstratos

Full decomposition of DTstratos

• DTstratos is NOT dominated by SWV• Other radiative effects, esp. those from

troposphere, offset SWV radiative cooling of stratosphere!

Stratos WV Tropos WV

Tropos Temp Surf Temp

Surf. Albedo Strato Dynamics

Clouds Residual

DTstratos change driven by different perturbations obtained via radiative equilibrium simulations [K]

Radiative feedback of DTstratos

• DTstratos is NOT dominated by SWV• Other radiative effects, esp. those from troposphere, offset SWV radiative cooling of

stratosphere and thus neutralize the SWV warming effect!

DR change driven by different perturbations

SWV DT StraQ

DTTropQ

DTTropT

DTTS

DTAlbedo

DTDyn

DTCloud

Flux Changes at TOA(W m-2)

0.23 0.53 0.19 -0.56 -0.18 0.03 -0.21 0.17

Feedbacks (W m-2 K-1) 0.05 0.12 0.04 -0.12 -0.04 0.01 -0.05 0.04

Accounted by FDH NOT accounted

Surface warming effect of SWV?

Global mean DTS, CESM 4xCO2

• How to determine the surface warming effect of SWV?

- RTM: Instantaneous forcing [Huang et al. 2016]

- FDH: Stratosphere-adjusted forcing [Banerjee et al. 2019; Dessler et al. 2013; Solomon et al. 2020; …]

- Mechanism denial experiment: SWV-locking

Control: 1xCO2Standard: 4xCO2, CESM (CAM+SOM)Locking: 4xCO2, SWV replaced with

Control values

Rnet = F + lDTS = 0F = 8.4 W m-2 (from fixed-SST 4xCO2) DTS = 7.7 K=> l = -1.1 W m-2 K-1

Had lSWV= 0.3 W m-2 K-1 been subtracted,DTS(no SWV feedback) = F/-(l-lSWV) = 6.1 K

7.7

6.1

Surface warming effect of SWV?

• How to determine the surface warming effect of SWV?

- RTM: Instantaneous forcing [Huang et al. 2016]

- FDH: Stratosphere-adjusted forcing [Banerjee et al. 2019; Dessler et al. 2013; Solomon et al. 2020; …]

- Mechanism denial experiment: SWV-locking

Control: 1xCO2Standard: 4xCO2, CESM (CAM+SOM)Locking: 4xCO2, SWV replaced with

Control values

Insignificant (2%) warming enhancement by SWV!

Rnet = F – lDTS = 0F = 8.4 W m-2 (from fixed-SST 4xCO2) DTS = 7.7 K=> l = -1.1 W m-2 K-1

Had lSWV= 0.3 W m-2 K-1 been subtracted,DTS(no SWV feedback) = F/-(l-lSWV) = 6.1 K

However, from SWV-locking: DTS= 7.5 K !

7.7

6.1

7.5

Global mean DTS, CESM 4xCO2

Small SWV warming: why?

• TOA budget perspective- Stratospheric cooling- Tropospheric warming- Cloud (high)

SWV coupled with other feedbacks;Compensation of these feedbacks neutralize the warming effect of SWV in (Stand-Locking) experiment.

dTs[K]

dTa[K]

d[log2(q)]

d(Cld)[%]

d(Alb)

Standard LockingStand -Locking

Zonally averaged climate feedback responses.

Small SWV warming: why?

• TOA budget perspective- Stratospheric cooling- Tropospheric warming- Cloud (high)

∆𝐓𝐬𝐮𝐫(K)

Forcing (W m-2)

∆𝐑𝐗 cause by Feedback Variables(W m-2)

Total(W m-2)

CO2, ins.

CO2, adj.

Wstr Wtro Tstr Ttro Tsur ALB CLD

Standard 7.71 4.70 3.67 0.18 12.95 -0.23 -21.31 -4.91 2.09 2.69 0.02

Locking 7.53 4.70 3.67 0.00 12.77 -0.77 -20.58 -4.75 2.05 2.97 0.03

Difference 0.18 0.00 0.00 0.18 0.18 0.54 -0.73 -0.16 0.04 -0.28 -0.01

SWV coupled with other feedbacks;Compensation of these feedbacks neutralize the warming effect of SWV in (Stand-Locking) experiment.FDH assessment may be misleading!

Global mean surface warming and TOA radiation flux changes, decomposed to forcing and feedback components using the kernel method.

Take-home messages• Strong stratospheric cooling due to SWV hypothesized by FDH

method: NOT observed because of compensating effects. • Surface warming effect of SWV assessed by a mechanism-denial, SWV-

locking experiment: small (2% warming enhancement).• No evidence of a strong stratospheric radiative feedback in GCM.

References• Huang, Y. and Y. Wang, (submitted), Stratospheric water vapor feedback

disclosed by a locking experiment, Geophys. Res. Lett.• Wang, Y. and Y. Huang, (submitted), Stratospheric radiative feedback

limited by the tropospheric influence in global warming, Climate Dynamics.